In recent years, pressure-sensitive adhesives have been used to bond, to a liquid crystal cell, an optical member such as a polarizing plate or a retardation plate for use in an electronic part such as a liquid crystal display device. Acryl-based pressure-sensitive adhesives have been widely used as such pressure-sensitive adhesives, because they have a high level of heat resistance and light resistance.
Unfortunately, acryl-based pressure-sensitive adhesives have not been able to exhibit high peeling resistance like adhesives do.
A thermosetting adhesive sheet capable of being cured by heat treatment has also been proposed for applications where electronic parts are bonded. Such a thermosetting adhesive has a high level of adhering strength and heat resistance but has no tackiness for adhesion at room temperature, which makes it difficult to achieve temporary bonding. In addition, before curing, the adhesive component contains a relatively large amount of a low molecular weight component, which may cause a problem such as oozing during bonding. Therefore, there has been a demand for an adhesive sheet that can exhibit moderate adherability to serve as a pressure-sensitive adhesive at the initial stage of bonding to the adherend and can also exhibit tackiness and heat resistance as high as those of an adhesive after bonding.
For example, there has been proposed a curable thermo-bonding sheet including an acrylic polymer, an epoxy group-containing resin, a photo-initiator, and a tackifier resin (Patent Document 1). Unfortunately, such a sheet contains a low molecular weight epoxy resin, and therefore, it is difficult to prevent oozing and the like when temporary bonding is performed using such a sheet.